1. Field of the Invention
The present invention relates to a distributed feedback (DFB) type semiconductor laser device such as a xcex/4-shifted DFB type semiconductor laser device.
2. Description of the Related Art
In a xcex/4-shifted DFB type semiconductor laser device, a xcex/4 shift is located at the center of a waveguide, which can theoretically realize 100 percent yield single mode characteristics. If such a xcex/4-shifted DFB type semiconductor laser device has an anti-reflection (AR) coating for outputting a signal light on the front side and an AR coating for outputting a monitoring light on the rear side, the output power of the signal light is theoretically the sane as the output power of the monitoring light. As a result, the light output characteristics of the signal light deteriorate.
In order to suppress the above-mentioned light output characteristics, in the prior art, a xcex/4 shift is located on the rear side from the center of the waveguide (see JP-A-3-110885). This will be explained later in detail.
In the above-described prior art device, however, the coupling coefficient is rapidly changed in the waveguide. Therefore, the output power ratio of the signal light to the monitoring light has a severe trade-off relationship with the normalized threshold gain difference and the deviation of the xcex/4 shift location. As a result, a margin or tolerance of the ratio of coupling coefficient on the front side to the coupling coefficient on the rear side is very small, which decreases the manufacturing yield.
In addition, a rapid change of the diffraction grating at the xcex/4 shift location invites a rapid change in the equivalent refractive index of an optical waveguide layer, which deteriorates the single mode characteristics.
Further, in a high output mode, the Bragg oscillation condition is changed by the spatial hole burning phenomenon, so that a mode skip occurs.
Additionally, in the case where the phase shift location is definite, when the coupling coefficient ratio is changed, the normalized threshold gain difference greatly changes, so that stable single mode characteristics cannot be realized.
It is an object of the present invention to provide a DFB type semiconductor laser device capable of increasing the manufacturing yield, improving the single mode characteristics, and avoiding a mode skip due to the change of the Bragg condition.
According to the present invention, in a DFB type semiconductor laser device having a waveguide for outputting a signal light from a front side of the waveguide and a monitoring light from a rear side of the waveguide, an active layer is formed above a semiconductor substrate, and an optical guide layer having a diffraction grating is provided. A coupling coefficient of the waveguide is gradually increased from the front side of the waveguide to the rear side of the waveguide.